...warm or cold?
#Roofs can be in every aspect complex design components, which come in various shapes and forms depending on the overall design, construction method and location. Regardless of the type of the roof, they serve a common purpose and that is to act as a protective cover for the inside of a building and for the structure itself.
Considering how the roof is going to be built from an incipient design stage would ensure that there are no issues at #construction stage such as exceeding the height limits imposed by Planning nor the headroom required by the client to comply with Building Regulations’ energy efficiency or fire safety requirements.
Since roofs are ubiquitous in architecture work, I thought I would share my knowledge and experience on the subject, as I consider that understanding how each of these roof systems works is fundamental to ensure a roof can perform its function.
Regardless of the type – warm or cold - a roof needs to perform its main roles of dispersing rainwater and making a building watertight, while adding to the thermal efficiency and achieving a certain level of fire protection.
Although traditional roof construction has gone a long way to provide diverse options for various design needs, we can still recognize the major ‘players’ preferred by #architects and specifiers due to their proven #performance and longevity, which can be narrowed down to either the #warm or the #cold roof. Slight variations such as the #hybrid or the #inverted roof are also valid options preferred by the industry, but since they derive from the two types mentioned above and for the purpose of this article, I will detail further on the former.
To start with, I had trouble myself distinguishing between a warm and a cold roof at some point, making it difficult for me to fully understand what the design of each of them would entail, despite the clear distinction between the two. In simple terms, the differences between a warm and a cold roof are the #ventilation #requirements along with the position of the insulation relative to the ‘deck’, which is usually above for the warm roofs and below for the cold ones. However, this can become quite complicated, since there are multiple possibilities for the insulation in the cold roof scenario, as this can be fully or partially fitted in between the #rafters, can be combined with insulated plasterboard or laid flat at ceiling level, depending on how the roof space is being used.
Strictly speaking, the roofs that are referred to as cold roofs have the structure exposed to cold, since no insulation goes on top, this being usually fitted at or below the rafters zone.
As a result, there is a risk of #condensation occurring, so suitable ventilation is required to dry any #moisture trapped in the roof space. Depending on the size and the shape of the roof, the design should cater for ventilation around the eaves, the ridge or wall #abutments so the air can flow and dry off any moisture. At this point, it is important to understand what is being ventilated, as in the case of a cold flat roof, it is the area between the deck and the insulation that is required to maintain a 50mm clear air void, as in the #sketch #detail below.
For a #pitched roof, outside air is required to flow inside the roof space, so provisions should be made to bring the air in below the #sarking membrane, as in the schematic drawing for a cold ventilated roof below. Additional ventilation is required between the sarking membrane and the roof covering to vent the tile battens. Although a 25mm ventilation void is usually sufficient, this should be increased to 50mm, by means of counter battens, should tight-fitting roof coverings such as fibre cement #slates or shingles are being used, as pictured in the #mansard edge roof sketch detail below. As such, the roof covering manufacturers should be able to advise on the ventilation requirements and suitable proprietary ventilators.
As mentioned above, a warm roof would not need any ventilation provision, as the purpose of the insulation above the deck is to create a weathertight enclosure, as pictured in the warm flat roof abutment detail below. This way, the roof would be as ‘warm’ as the space below, hence the name.
Although warm roofs require no ventilation whatsoever within the roof space, there are instances such as in the sketch detail of a ventilated warm pitched roof below, where there should be a 50mm ventilation gap above the #LR #underlay to ensure that any water vapours coming from below are dried. Therefore, it is good practice to seek advice from the roof insulation manufacturer to understand the ventilation requirements required for a particular buildup.
By its nature, a roof carries a lot of weight, as built and in the #design #process, so this should be given proper thought from the beginning of the project. Regardless of the type – warm or cold - a roof needs to perform its main roles of #dispersing rainwater and making a building #watertight, while adding to the #thermal efficiency and achieving a certain level of #fire #protection. So long as #designers grasp the #principles of distinct types of roof construction, they can leverage their knowledge to design a roof that performs as intended and corresponds to the #client’s brief.
As I said at the beginning of this article, a roof is a complex element and there are multiple #solutions depending on various factors, way beyond the few generic details I used to illustrate how some roofs work, thus I would be curious to know what roof type would you go for in your designs? Feel free to share your thoughts on this in the comments section.
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